The present invention relates to positioning systems. More specifically, the present invention relates to systems used to identify the locations of or track objects in a given area.
An Indoor Positioning System (IPS) is a network of devices used to wirelessly locate objects or people inside a building or within dense industrial areas. A special design is required since global positioning system (GPS) systems are typically not suitable to establish indoor locations as they require an unobstructed line of sight to four or more GPS satellites. Microwaves will be attenuated and scattered by roofs, walls and other objects and multiple reflections at surfaces cause multipath propagation serving for uncontrollable errors.
Time of flight (ToF) is the amount of time a signal takes to propagate from transmitter to receiver. Because the signal propagation rate is constant and known, the travel time of a signal can be used directly to calculate distance. Multiple (in GPS at least four satellites) measurements vs. multiple anchor stations can be combined with trilateration to find a location.
As speed of light is 3×108 m/sec, in radio frequency (RF) based systems, inaccuracy in clock synchronization is a key factor of the positioning error. In GPS, ToF generally requires a complicated synchronization mechanism to maintain a reliable source of time for sensors.
In addition, the accuracy of the ToF based methods often suffer from large multipath conditions in indoor localization, which is caused by the reflection and diffraction of the RF signal from objects.
Due to the attenuation and reflections caused by construction materials, it is desirable to have an unobstructed line of sight to at least three anchor points at any location that should be covered by the system. As a result, a larger number of anchor stations are required.